How do I bring up the Letters on a key pad when I am putting in an accout number?

These days when calling on a Galaxy S3 or perhaps any phone, if asked for the account number you are only able to put in numbers on the key pad. My account had a letter and I could not figure out how to do that on my phone. Therefore, I started over and over. Please tell me how to make the key pad on my phone do a letter instead of a number.

Contact whoever the account is with and ask them how they want it done. Telephone keypads produce tones, not numbers or letters, and the only tones they produce are interpreted (at the other end) as numbers. They may want you to press the key for the number that the letter is on (like 3 for D, E or F, or maybe press it the number of times to get to the letter (like 33 for E, 333 for F, etc.)

If they want you to put in an actual letter with a telephone keypad, they need a clue, since they evidently don't have any of their own.